Retention & Drainage Aid

PAM for Paper Making Retention & Drainage

Retention and drainage are two outcomes of the same wet-end decision. A retention aid holds filler and fines in the sheet instead of losing them to white water; a drainage aid lets water leave the wire faster so the sheet arrives at the press section drier. High molecular weight polyacrylamide does both, because the bridging flocs it forms are simultaneously large enough to be captured by the fibre mat and open enough to let water pass. That is why one chemical family sits at the centre of a problem that mills usually describe as two separate problems.

The trade-off is sheet formation. Push flocculation hard enough and first-pass retention climbs while formation deteriorates — visible as a cloudier look-through, higher basis weight variation and, on printing grades, uneven ink lay. Every number on this page exists inside that constraint. We supply paper-grade cationic and anionic PAM from our own plant in Xinxiang, Henan, with 100,000 tons/year of PAM capacity, and we would rather help you find the dose that holds formation than sell you the dose that maximises retention on paper.

What the Polymer Actually Does at the Wet End

Furnish components — refined fibre, fines, clay, precipitated or ground calcium carbonate, titanium dioxide — carry a net negative surface charge and repel each other. Fines and filler are small enough to follow water through the forming fabric rather than being trapped by it. Two distinct mechanisms recover them.

Charge patching uses a low molecular weight, high charge density cationic polymer to create positive patches on negative particle surfaces. Particles then attach to each other patch-to-bare-surface. Patching gives small, shear-resistant flocs, so it protects formation but recovers less material.

Bridging uses high molecular weight PAM whose extended chains adsorb onto several particles at once and physically tie them together. Bridging recovers far more filler and drains much faster, but the flocs are large and shear-sensitive. Once a bridged floc is broken by a pump or a screen it does not fully reform, because the chain segments that were available for adsorption are now lying flat on the particle surface. This irreversibility is the single most important practical fact about wet-end PAM, and it dictates where the polymer goes in — see the addition point section below. The relationship between chain length, charge and floc behaviour is covered in more depth in our molecular weight guide and charge density explainer.

Drainage improves for a second, less obvious reason. Water held between individual fines particles is immobilised by capillary forces. Aggregating those fines into flocs collapses many small channels into fewer large ones, and flow through a porous mat scales with the square of channel radius. That is why a modest retention gain often produces a disproportionate drainage gain, and why chasing drainage with vacuum alone is expensive compared with chasing it chemically.

Single Polymer, Dual Polymer and Microparticle Systems

Microparticle systems win on the formation axis because the reflocculation after shear is genuinely reversible — the inorganic particle re-bridges polymer-coated fibres downstream of the pressure screen. If you run a fast machine on a bright grade and cannot tolerate look-through loss, that is the architecture to aim at, and PAM is one component of it rather than the whole answer. On packaging and board, where formation tolerance is wider and cost pressure is harder, single CPAM usually delivers the better return.

Grade Selection by Paper Type

Read the units carefully. Wet-end retention doses are kilograms per tonne of oven-dry fibre and land in the 0.2-1.2 kg/t band — a few hundred grams of polymer per tonne of paper. The dewatering row is kilograms per tonne of dry solids in the sludge, an order of magnitude higher on a completely different basis. Mixing the two bases is the most common costing error we see in enquiries, and it is easy to overstate polymer spend by 10× without noticing. Our dosage calculation walkthrough sets out the arithmetic both ways. Full specifications for the grades in the table above are on our cationic polyacrylamide and anionic polyacrylamide product pages. For the effluent and saveall side of the mill, the grade selection logic is different again and is covered in PAM for paper mill wastewater.

The Addition Point Rule Most Mills Get Wrong

Because bridged flocs do not reform after high shear, a high molecular weight retention aid must be dosed after the last high-shear element in the approach flow — that means after the fan pump and, on most machines, after the pressure screen, as close to the headbox as mixing quality allows. Dosing a bridging CPAM before the fan pump is the single most common reason a mill concludes "the polymer does not work" when in fact the polymer worked and was then destroyed 40 metres upstream of the wire.

The exception is charge control chemistry. Fixatives and coagulants work by neutralising charge, not by bridging, so they are unaffected by shear and belong early — machine chest or fan pump suction — where they have residence time to react with dissolved and colloidal material. The general sequence is: fixative early, starch mid, bridging polymer late, microparticle last.

Mixing quality at the injection point matters as much as its position. Inject into a turbulent zone through a multi-port injection quill or a dilution ring rather than a single nozzle into a slow-moving header. Poorly distributed polymer produces local overdose streaks — visible as spot flocs and formation defects — while the rest of the furnish stays under-dosed, giving you the worst of both outcomes at full chemical cost.

When Rising CPAM Demand Is Not a Polymer Problem

If your CPAM dose has crept up over months while retention has stayed flat, the polymer is probably not the variable that changed. Dissolved and colloidal substances — lignin fragments, hemicelluloses, stickies, surfactant residues, collectively "anionic trash" — consume cationic charge before it ever reaches a filler particle. Every kilo of CPAM you add is being spent on neutralising the water rather than on holding the sheet together.

The diagnostic is a cationic demand titration on filtered white water, trended over time rather than read once. If demand is rising, the correct response is a cheap low molecular weight fixative dosed early — polyDADMAC or polyamine — to strip the charge load, followed by a normal, low dose of high molecular weight CPAM to do the actual bridging. Substituting expensive high-MW polymer for cheap charge control is a costly way to solve a problem it is badly suited to. Our polyDADMAC guide covers the fixative side; the split of duties between anionic and cationic grades is laid out in anionic vs cationic polyacrylamide.

Closing up the water loop makes this worse in a predictable way. Every reduction in fresh water intake concentrates the dissolved load in white water, so a mill that cuts specific water consumption without adding charge control will watch its polymer bill rise and blame the supplier. That is a system change, not a chemistry failure.

Measuring Retention and Drainage Honestly

Four measurements decide whether a polymer trial succeeded. Run all four, because any one of them alone can be improved while the machine gets worse overall.

• First-pass retention — headbox consistency versus white water consistency. This is the number a retention aid is bought to move, and it should be read as a trend across a shift rather than from a single grab sample.
• First-pass ash retention — the same calculation on ash content instead of total solids. On filler-loaded grades this is where the money is, and it usually moves further than total retention does.
• Drainage rate — freeness or a dynamic drainage measurement. A shear-and-drain test that reproduces machine shear is far more predictive than a static one, because it exposes exactly the floc fragility discussed above.
• Formation — the veto measurement. Look-through, basis weight variation, formation index. If formation degrades, the retention gain is not a gain; it is a quality problem you have not been charged for yet.

Bench screening comes first. A properly run jar test on actual furnish narrows four candidate grades to one in an afternoon and costs nothing but time, which is why we send samples for it rather than asking mills to trial a full tanker. The procedure, including the shear step that most operators skip, is in our jar test procedure. Broader wet-end context for paper-grade selection sits in polyacrylamide as a paper making retention aid.

Worked Example: Drainage-Limited Tissue Machine

Modelled from process arithmetic, not a delivered project. A tissue machine is held at 800 m/min because the sheet reaches the Yankee too wet — the constraint is drainage on the wire, not dryer capacity. Baseline first-pass retention is around 72%, so roughly 28% of the furnish is recirculating through the saveall and back into the approach flow, where the accumulated fines slow drainage further. It is a self-reinforcing loop.

Move first-pass retention to about 85% with a CPAM in the 8-12M MW range at 0.4 kg/t, dosed after the pressure screen, and two things change at once: less fines mass recirculates, and the fines still present are aggregated rather than free. Both open drainage channels in the mat. On a machine where drainage is genuinely the binding constraint, a ceiling near 950 m/min is a realistic expectation for that shift — call it 15-18% more output from a chemical change costing a few hundred grams of polymer per tonne.

The honest caveat is the conditional. If your real limit is the dryer, the vacuum system or the press, the same retention improvement will show up as a small steam saving and nothing else. Establish which element is actually binding before budgeting for the speed gain, because the arithmetic above only holds for the drainage-limited case.

Wet-End Dosing Practice

1. Make down at 0.2-0.5% and post-dilute to 0.05-0.1% at the injection point. Dilute polymer disperses before it flocculates; concentrated polymer flocculates before it disperses.
2. Give powder grades 30-60 minutes of ageing. Under-hydrated PAM has not reached its working chain extension, so you pay for molecular weight you never use.
3. Use low-shear transfer. Progressive cavity or diaphragm metering pumps, not centrifugal. Chain scission in the dosing line is invisible and permanent.
4. Change one variable per shift. Dose, addition point and grade all move retention. Changing two at once produces a result you cannot attribute or repeat.
5. Watch for the overdose turn. Past the optimum, charge reversal redisperses flocs — white water consistency climbs again and formation worsens. If more polymer is making things worse, the answer is less polymer, not a different polymer.
6. Hold the supplier to batch consistency. We test every batch for molecular weight to a ±0.5M tolerance and retain samples for 24 months, so when performance shifts we can compare your drum against the retained sample and establish whether the variable was the polymer or the furnish.

Standards and Test Methods We Work To

Wet-end chemistry is assessed against published methods rather than supplier claims. The relevant bodies for this application are TAPPI, which publishes the standard test methods for freeness, ash content and formation used across the industry; ISO, whose paper and board testing standards govern most export specifications; Zellcheming, whose test methods are widely referenced by European mills; and PITA, which publishes applied wet-end research of direct use to machine crews. For food-contact and hygiene grades, residual acrylamide monomer limits are set by the relevant national food-contact regulation in your market, and we supply certificates of analysis showing monomer content per batch so that compliance can be demonstrated rather than asserted.

We deliberately do not cite document numbers for these methods on this page. Method designations are revised, and a stale number quoted with false precision is worse than a named body you can look up. Ask us for the current designation against your specific test and we will confirm it in writing.

Frequently Asked Questions

Where exactly should retention aid be dosed on the approach flow?

After the last high-shear element — normally after the pressure screen, as near the headbox as adequate mixing allows. High molecular weight bridging flocs do not reform once broken, so polymer dosed before the fan pump is largely wasted. Charge-control fixatives are the opposite: they belong early, because they need contact time and are indifferent to shear.

My CPAM dose keeps rising but retention is flat. Why?

Almost always rising anionic trash in the white water, consuming cationic charge before it reaches filler. Titrate cationic demand on filtered white water and trend it. The fix is a cheap low molecular weight fixative dosed early plus a normal low dose of high-MW CPAM, not more high-MW CPAM.

Should I use cationic or anionic PAM for retention?

Cationic as a single-polymer system, because furnish is negatively charged and needs charge interaction to start aggregation. Anionic high-MW PAM is used as the second component behind a cationic fixative or starch, where the charge work is already done and the polymer only has to bridge. Anionic alone on unmodified furnish does very little.

Will a retention aid hurt my sheet formation?

It can, and that is the real limit on dose rather than cost. Monitor formation alongside retention and stop at the dose where look-through starts to degrade. If you need retention beyond that point, change the system architecture to a microparticle programme rather than pushing the same polymer harder — reversible flocs give you retention without the formation penalty.

What dose should I budget per tonne of paper?

For wet-end retention, 0.2-1.2 kg per tonne of oven-dry fibre depending on grade and furnish, with filler-loaded fine paper typically 0.3-0.8 kg/t. Do not confuse this with sludge dewatering dosing, which is 3-8 kg per tonne of dry solids on an entirely different basis. Confirm with a jar test on your own furnish before budgeting.